package: Makefile: unbreak binrpm-pkg target

Commit 6501320311664e11ad5489fedb17ea1f817c2617 dropped the rpm spec as a
prerequisite for the binrpm-pkg target but forgot to update $< usage,
which causes the rule to break.

This commit fixes that by replacing $< with the spec name.

Signed-off-by: Luiz Capitulino <lcapitulino@redhat.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>